The invention relates to an exposure control system primarily, though not exclusively for an electro-statographic copy machine.
In the practice of xerography, as described for example, in U.S. Pat. No. 2,297,691 to Chester F. Carlson, a xerographic surface comprising a layer of photoconductive insulating material affixed to a conductive backing is used to support electrostatic images. In the usual method of carrying out the process, the xerographic plate is electrostatically charged uniformly over its surface and then exposed to a light pattern of the image being reproduced to thereby discharge the charge in the areas where light strikes the layer. The undischarged areas of the layer thus form an electrostatic charge pattern in conformity with the configuration of the original light pattern.
The latent electrostatic image can then be developed by contacting it with a finely divided electrostatically attractable material such as a powder. The powder is held in image areas by the electrostatic charge on the layer. Where the charge is greatest, the greatest amount of material is deposited; and where the charge is least, little or no material is deposited. Thus, a powder image is produced in conformity with the light image of the copy being reproduced. The powder is subsequently transferred to a sheet of paper or other surface and suitably affixed thereto to form a permanent print.
It can be readily appreciated that the quality of the print is, in the large part, dependent on the exposure of the charged xerographic plate to the radiation image. The largest single factor affecting exposure latitude, i.e., range of illumination intensity, is the efficiency of the developer system. In other words, if the developer system is highly sensitive so as to develop background or image portions as "grey" areas when in reality these are white, then illumination control must be commensurately sensitive to provide the proper exposure of the charged xerographic surface. With modern-day improvement to xerographic developer systems, the desirability of maintaining proper illumination becomes increasingly apparent.
A uniformly high level of illumination as required for exacting exposure is complicated by many factors. For example, variation in lamp output due to lamp aging or deterioration is sufficient to cause development of white areas thereby detracting from overall quality of the print. It has been determined, for example, that deterioration of some lamps is dependent on properties of their phosphor coating. The deterioration characteristics of aperture lamps having the same type of phosphor coating do not differ significantly. The deterioration of certain types of such lamps can be as much as 40% after approximately 1,000 hours of use. Such a large change in illumination level cannot be tolerated in modern copying systems. Thus, such lamps are generally replaced after a time period much earlier than the 1,000 hours' deterioration period mentioned hereinabove.
Some prior art proposals for compensating for variation in lamp output utilize photosensitive devices, such as photocells, which measure lamp output and adjusts various machine parameters to compensate for the variation in lamp output. The present invention relates to and is particularly suitable for providing intensity of illumination compensation in an electrostatographic machine which includes a scanning optical system.
A copying machine such as that described in British Patent specification 995413 uses an optical system comprising two fixed mirrors with a lens between the mirrors, the stationary original being illuminated by lamps on a movable lamp carriage, and light from the original being screened from the projection system except for a small slit between the lamps. British Patent specification 1,122,622 describes a document copying machine in which the whole of the stationary original is illuminated during exposure, and scanning is achieved by oscillating one of the mirrors of the projection system about an axis in its plane. It has been proposed, for example, in the U.S. Pat. No. 3,642,366 to have a more compact image projection system in which two mirrors are moved in different directions at speeds relating to the speed of movement of the photosensitive surface.
In copiers having optical systems of the kind already mentioned and generally any copiers relying on a photosensitive response, it may be desirable to adjust the illumination of the document to be copied to maintain as far as possible a constant irradiance at the image plane, that is at the photosensitive surface of photoreceptor. This constant irradiance is desirably achieved for various original document background reflectivities and as far as practical in some cases, despite aging or other forms of deterioration of the optical system, and deterioration of lamps.
In many copiers, copies provided depend for their definition on the difference of light intensity between the light and dark part of the original document to be copied so that adjustment of the illumination of the document may not be so critical. However, in a copier as described in U. K. Pat. No. 880,597 the definition of the copies made depends, in effect, on the actual value of the illumination received, rather than a differential value, so that ensuring near-constant irradiance received at the photoreceptor surface for differing types of original, that is, for each individual original, becomes even more important.